The US Astro2020 decadal panel has selected three Probe-scale space mission candidates
to be implemented over the 2020s and early 2030s. A cosmic microwave background imaging
polarimeter is one of the candidates. PICO is a concept for such a space mission, and it
was competitively selected by NASA for study in preparation for Astro2020. PICO employs
a focal plane with 13,000 detectors spread among 21 frequency bands. Over a 5 year
mission scanning the entire sky it will produce maps with a depth of at least 0.87
microKelvin arcmin, equivalent to more than 3300 planck missions. I will describe PICO's
science goals, required performance, and recent work to validate the requirements.